(scm_string_any, scm_string_every,

(scm_string_any, scm_string_every,
scm_string_tabulate, string_upcase_x, string_down_case_x,
string_titlecase_x, string_reverse_x, scm_string_tokenize): Use
size_t instead of int for indices into strings.  Make sure that no
over- or underflow occurs.  Thanks to Andreas Vögele!
(scm_xsubstring, scm_string_xcopy_x): Use ints for 'extended'
indices, which can also be negative.